1. Field of the Invention
The invention relates to a method of estimating the relative frequency uncertainty between a part of a mobile radio system able to use a first mobile radio network at a first particular frequency and another part of the same mobile radio system able to use a second mobile radio network at a second particular frequency.
2. Description of the Prior Art
The field of the invention is that of mobile radio.
At present, most mobile radio calls are transmitted over a single network, such as the GSM network. New generations of networks are now appearing, for example the UMTS network, which coexist with previous networks, if only during a transition period.
It is necessary to be able to communicate in two different transmission modes, i.e. over two different networks, using the same mobile radio system, for example the same mobile telephone. The GSM and UMTS networks are used as examples hereinafter, but the invention applies equally well to other networks.
A mobile radio network includes base stations which send to and receive from mobile telephones in their coverage area in a particular frequency band.
Communicating via two different networks consists in being able to communicate interchangeably via the GSM network or the UMTS network as a function in particular of the GSM or UMTS base station in whose coverage area the mobile telephone is located and in being able to start a call via the GSM network and continue it via the UMTS network, for example because the mobile has moved from the coverage area of a GSM base station to that of a UMTS base station.
For this to be possible the mobile telephone must be synchronized to the GSM network and to the UMTS network at all times. Synchronization includes time synchronization, which entails knowing the time on the GSM network and the time on the UMTS network, and frequency synchronization.
The remainder of the description concerns frequency synchronization.
Base stations and mobile telephones include a quartz crystal clock from which particular communication frequencies are derived.
Frequency synchronization entails knowing the relative uncertainty of the different clock frequencies used in a call, caused by drift affecting the clocks.
FIG. 1 shows diagrammatically a mobile radio system with two networks. It includes a GSM base station 1, a UMTS base station 2 and a mobile telephone 3 having a GSM part 31 and a UMTS part 32.
Four frequencies are used in a call using the GSM network and the UMTS network: the frequency fGSMBase of the GSM base station 1, the frequency fUMTSBase of the UMTS base station 2, the frequency fGSMMobile of the GSM part 31 of the mobile, and the frequency fUMTSMobile of the UMTS part 32 of the mobile.
Some uncertainty as to these frequencies is tolerated, enabling a required quality of service to be maintained:
with regard to the base station frequencies, GSM recommendation 05.10 V6.3.0 paragraph 5.10 specifies that the relative difference dGSM between the nominal GSM frequency and the frequency fGSMBase must not exceed 0.05 ppm (parts per million), and likewise the relative difference dUMTS between the nominal UMTS frequency and the frequency fUMTSBase:|dGSM|<0.5 ppm and |dUMTS|<0.5 ppm
As a result of this, the uncertainty or tolerance ΔBase associated with the frequencies of the GSM base station 1 and the UMTS base station 2 must not exceed 0.1 ppm:|ΔBase|<0.1 ppm, where      Δ    ⁢                   ⁢    Base    =                              f          GSMBase                -                  f          GSMnom                            f        GSMnom              +                            f          UMTSnom                -                  f          UMTSBase                            f        UMTSnom            
also, if a call is set up between the UMTS base station 2 and the mobile 3, in this instance the UMTS part 32 of the mobile, the relative uncertainty ΔUMTS between the frequency fUMTSBase and the frequency fUMTSMobile can be measured, and is expressed by the following equation:       Δ    ⁢                   ⁢    UMTS    =                    f        UMTSBase            -              f        UMTSMobile                    f      UMTSnom      
no call being set up between the GSM base station 1 and the mobile 3, in this instance the GSM part 31 of the mobile, the relative uncertainty ΔGSM corresponding to the frequencies fGSMBase and fGSMMobile is not known a priori, and is expressed by the following equation:       Δ    ⁢                   ⁢    GSM    =                    f        GSMMobile            -              f        GSMBase                    f      GSMnom      
likewise, if ΔMobile designates the relative uncertainty associated with the frequencies fUMTSMobile and fGSMMobile, ΔMobile is expressed by the following equation:       Δ    ⁢                   ⁢    Mobile    =                              f          UMTSMobile                -                  f          UMTSnom                            f        UMTSnom              -                            f          GSMnom                -                  f          GSMMobile                            f        GSMnom            
Also: ΔBase+ΔUMTS+ΔMobile+ΔGSM =0
The problem is to estimate the uncertainty ΔGSM in order to be ready to communicate over the GSM network from the very start of the call.
A solution that measures the uncertainty ΔGSM by setting up a GSM call is time-consuming and interferes with other calls until the uncertainty ΔGSM has been measured.
ΔBase being limited, and ΔUMTS being known, the object of the present invention is to calculate ΔMobile to determine ΔGSM.
The frequencies fUMTSMobile and fGSMMobile are respectively derived from the clock of the UMTS part 32 and the clock of the GSM part 31, whose signals are shown in FIG. 2. As a result of this, any frequency offset that may exist between the two mobile telephone clocks causes a shift between the frequencies of the mobile corresponding to the uncertainty ΔMobile.
This shift is conventionally calculated by the counters method, i.e. by counting the number NUMTS of clock pulses of the part 31 and the number NGSM of clock pulses of the part 32 from a time t during a particular time window of duration T. The duration T required for the numbers NUMTS and NGSM to be sufficiently accurate is generally of the order of 1 second. According to some calculations, T=1.04 s.
If neither the frequency of the clock of the GSM part 31 nor the frequency of the clock of the UMTS part 32 is subject to any shift, and these frequencies are therefore respectively equal to 13 MHz and 19.2 MHz, the ratio NUMTS/NGSM is constant; to be more precise:             N      UMTS              N      GSM        =            f      UMTSMobile              f      GSMMobile      
If a shift occurs at one of the clocks, and therefore in the corresponding frequency then:                     N        UMTS                    N        GSM              =                            f          UMTSMobile                          f          GSMMobile                    +      Δ        ⁢         
The term Δ linked to the existing frequency shift between the two clocks of the mobile telephone is the required ΔMobile. It can be obtained by the counters method just described.
However, the period of 1.04 s over which the numbers NUMTS and NGSM of pulses are measured is very long on the scale of a mobile radio call.
Also, the current availability of the measured value of ΔMobile every 1.04 s is insufficient.
This is because ΔMobile varies during the measurement process, in particular as a function of temperature, humidity, supply voltage, surrounding electronics, etc: it is therefore useful to determine ΔMobile more frequently than every 1.04 s, i.e. at intermediate times, for example every 0.5 s, as in the GSM network.
The object of the invention is to propose a method of estimating frequency uncertainty that improves the availability of the uncertainty measurement by simultaneously measuring the GSM and UMTS counters at intermediate times that are much more closely spaced and storing the results obtained in memory.
Also, the accuracy of the measured value ΔMobile is insufficient.
In the final analysis, the result of the measurement is no more than an average over the period T, whereas what is required is a result at a given time t.
This problem is illustrated by an example shown in FIG. 3.
The measurement starts at a time tDM and ends at time tFM, i.e. 1.04 s later. An uncertainty ΔMobile (or ΔM) with linear variation is taken by way of example, but this does not always apply. The uncertainty ΔMM measured by the counters method is not the required uncertainty ΔM1 but the average uncertainty, i.e. ΔM0+(ΔM1−ΔM0)/2.
The object of the invention is to propose a frequency uncertainty estimation method that improves the accuracy with which ΔMobile is measured from the evolution of ΔMobile.